The use of intravenous administration systems incorporating 3-mm internal diameter tubing is becoming more common in hospital practice. The maximum flow-rate of crystalloid solutions through 3-mm-diameter tubing is compared to that through conventional 4-mm tubing when connected to standard large-gauge intravenous cannulae. Lengths of intravenous tubing between 80 cm and 200 cm were tested in combination with 16 gauge, 14 gauge and 13 gauge intravenous cannulae. The results demonstrated that the use of 3-mm-diameter infusion sets, or the inclusion of even short lengths of 3-mm tubing in an infusion system, limits the maximum flow that can be delivered through a cannula of size 16 gauge or larger. The reduced performance of the 3-mm tubing makes administration systems incorporating even short lengths of this diameter of tubing unsuitable in emergency situations and locations where rapid infusion of fluids is vital.